Overall I like Clickbank. They provide good tracking statistics for affiliates, they pay on time and they have a large number of products to promote. Yes, some of these are rubbish, as Marisa Wright says, but I have a policy of testing any product before I promote it. If it is rubbish I return it for a refund.
This unfortunately backfired on me some years ago when, by chance, I happened to return 3 products within a couple of weeks. The next time I tried to buy a product through Clickbank I found that they had blacklisted me and would not let me buy anything! So the moral here is: try before promoting, and return the rubbish VERY carefully.